March 2012 - Mississippi Geological Society

MGS LUNCHEON SPEAKER:
Sub-aerial Basins Below Sea Level Provide Unexpected Reservoirs
Cassidy, Martin M., Burke, Kevin
Department of Earth and Atmospheric Sciences
University of Houston, Houston, Texas 77204-5007
mcassidy@uh.edu
Abstract:
Throughout geologic history there have been large sub-aerial basins below sea level. There are two times in the plate tectonic cycle when
such basins are likely to form: in rifting of cratons, examples being the Afar (- 410ft, below sea level) Southern North Sea (-750 ft.), and the
South Atlantic basins; and when old basins are sealed off in collisions such as the Mediterranean during Messinian time (-10,000,ft.) the Black
Sea (-550ft.), and the Gulf of Mexico (GOM) during deposition of the Norphlet samds in Jurassic and perhaps in the Paleocene Wilcox sands
in basin depths (-6000ft.?) but in shallow water to subaerial.
Basins that were below sea level, but sub-aerial, influenced sedimentation and should influence interpretation of the tectonic history of the
basins. Sub-aerial sediments do not necessarily mean basin uplift!
A desiccated sub-aerial basin below sea level may be the site of extensive desert deposits. Winds pouring across the lip and down into a
subaerial basin below sea level are heated by compression as they descend leading to exstreme descication evaporating brines to even deposit
potassium salts. The same winds move sand dunes even into the deepest portions of the basin. Those potential reservoirs are not influenced by
distance from shore, as are marine sands. The most significant event in a sub-aerial sub-sea basin is the sudden flooding upon entry of the sea.
Unlike a marine transgression that reworks sediments on gradually submerged land, the sea fills the empty basin like filling a bathtub. The sea
rises to flood the surface with little disturbance covering the terrain in a geologic moment. Sand dunes are drowned, preserving their shapes
and cross bedding, as in Permian Rotliegend of the Southern North Sea. Porosity of sandstone may be preserved due to desert conditions leading
to chlorite overgrowths on quartz, as is true of Norphlet Sandstone of the GOM.
Canyons cut to grade with the basin floor are distinctive of former sub-aerial sub-sea basins. They bring coarse clastics to the basin floor.
Such buried canyons are found all around the Mediterranean and western Gulf of Mexico
After the flood mainly fine clay, carbonates, and organic matter settle out of the anoxic water. Rising H2S from rotting vegetation of the
suddenly drowned landscape precipitates metals from the ocean water and may cause metal rich fine sediments such as the Kupferschiefer that
overlies the sand dunes of the Rotliegend. The fines drape over the preexisting dunes like a blanket of snow, following the curves of the former
landscape.
In a basin containing a drowned desert environment can be found reservoirs that might not be expected if a uniformly marine basin model
is used in interpretation and exploration. Realization that one may be dealing with desert sedimentation can result in interpretations that extend
successful oil and gas plays and predict locations of new ones.
Martin M. Cassidy
Education:
Ph.D. Geology 2005 University of Houston.
M.S. Geology 1964 University of Oklahoma.
B.A. Geology cum Laude1955 Harvard University.
Experience:
Dec.2005-Present Research Scientist, Dept. of Geology, University
of Houston study of CO2 and Consultant Petroleum Geologist.
June 1994- 2005 Graduate student U. of H. and Consultant Petroleum Geologist.
1991 – 1994 Amoco International New Ventures regional assessment.
1988 - 1991 Amoco International New Ventures Indonesia.
1986 – 1988 Amoco Production Technical Advisor New Orleans.
1984 – 1986 Amoco (U.K.) Exploration Company Exploration Manager, London, England.
1982 - 1984 Amoco (U.K.) Exploration Company Division Operations Supervisor
1973 – 1982 Amoco International Exploration Company Various assignments including
New Ventures in Middle East, Far East Exploration operations Manager.
1969 – 1973 Pan American Libya oil Company Chief Geologist , Tripoli, Libya.
1962 – 1969 Pan American Petroleum Company, Geologist, South Texas oil and gas and
High Island sulfur exploration and development.
1960 – 1962 Harvard University Teaching Fellow.
1958 – 1960 M.S. University of Oklahoma.
1956 – 1958 U.S. Air Force Ammunition Supply Officer, Korea. Denver.
1955 – 1956 Standard of Texas, Geologist.